OFFSET
1,3
COMMENTS
In other words, the independence number of the (n-1)-triangular grid graph.
Also the independence number of the n-triangular honeycomb king graph. - Eric W. Weisstein, Sep 06 2017
LINKS
Colin Barker, Table of n, a(n) for n = 1..1000
A. V. Geramita, D. Gregory, and L. Roberts, Monomial ideals and points in projective space, J. Pure Applied Alg 40 (1986), pp. 33-62.
Stan Wagon, Graph Theory Problems from Hexagonal and Traditional Chess, The College Mathematics Journal, Vol. 45, No. 4, September 2014, pp. 278-287.
Eric Weisstein's World of Mathematics, Independence Number
Eric Weisstein's World of Mathematics, Triangular Grid Graph
Index entries for linear recurrences with constant coefficients, signature (2,-1,1,-2,1).
FORMULA
a(n) = ceiling(n(n+1)/6) for n > 5, see Geramita, Gregory, & Roberts theorem 5.4. - Charles R Greathouse IV, Dec 04 2014
G.f.: x*(x^9-2*x^8+2*x^7-3*x^6+3*x^5-2*x^4+2*x^3-2*x^2+x-1) / ((x-1)^3*(x^2+x+1)). - Colin Barker, Feb 08 2015
EXAMPLE
On a triangular grid of side 5 at most a(5) = 6 points (X) can be placed so that there is no pair of adjacent points.
X
. .
X . X
. . . .
X . X . X
MATHEMATICA
Table[1/18 (Piecewise[{{28, n == 2 || n == 4}}, 10] + 3 n (3 + n) + 8 Cos[(2 n Pi)/3]), {n, 0, 20}] (* Eric W. Weisstein, Jun 14 2017 *)
PROG
(PARI) Vec(x*(x^9-2*x^8+2*x^7-3*x^6+3*x^5-2*x^4+2*x^3-2*x^2+x-1)/((x-1)^3*(x^2+x+1)) + O(x^100)) \\ Colin Barker, Feb 08 2015
CROSSREFS
KEYWORD
nonn,easy
AUTHOR
Heinrich Ludwig, Mar 18 2014
EXTENSIONS
Extended by Charles R Greathouse IV, Dec 04 2014
STATUS
approved